Hierarchical structures composed of nodes and links can represent various decision logic. Decision logic can represent a series of decisions that divide a population into subsets. The decisions can be made on the basis of the values of one or more variables. Decision logic can be used to assign a single course of action (which could be to take no action at all) to each population subset. One example of decision logic is described in the U.S. Pat. No. 7,000,199, incorporated by reference hereto in its entirety.